Wind turbines are in use around the world to generate electric power. Large concentrations of wind turbines are installed in or near Tehachepi, Altamont Pass, and Palm Springs, Calif. as well as in various other locations in California and throughout the United States. Large concentrations of wind turbines are also located in Holland, Denmark, and Germany.
The rotating elements, or rotors, of wind turbines commonly comprise of three parts: a generator, a transmission, and an impellor which utilizes aerodynamic lift created by the passage of air over an airfoil to initiate and sustain a rotational force.
A common style of turbine has a horizontal shaft connecting the impellor to the transmission and the impellor is composed of a number of airfoils, or blades, protruding radially from a hub on the end of the horizontal shaft. The number of blades on a turbine can vary from two to five or more. The most common hub-blade assemblies utilize three blades fixed to the hub. The blades are typically attached to the hub by bolting a flange at the root of the blade to a mating flange on the hub. These flanges are held together by 20 or more hardened bolts. The manufacturers of both the hubs and blades have made no provision for field balancing of these rotors. Provisions have been made to statically balance sets of blades prior to installation onto a hub. But some residual unbalance remains even when static balancing of blade sets is properly performed. Furthermore, imperfections in the fabrication of hubs as well as age related factors relative to the blades themselves can lead to the need to field balance the rotors of wind turbines. It is desirable to have a balanced rotor because an unbalanced rotor can cause premature wear to turbine components. The components which are particularly susceptible to wear are the bearings which support the main horizontal shaft and the gear mechanism which yaws the turbine blades into the wind. Field balancing of the assembled rotors offers some distinct advantages to the equipment operator as compared to removing the rotor for balancing at a remote location. First, the hub with blades is a large and heavy structure which requires a crane or other heavy equipment for removal. Individual blades can weigh more than 800 lbs each. The hub can weigh 500 lbs or more. The entire assembly can weigh 3000 lbs or more. The diameter of the assembly can be greater than 50 feet. Removal of the hub assembly is costly and involves both a hazard for the equipment and the personnel engaged in such work. Furthermore, the blades are made of relatively light materials which can easily be damaged during the process of raising or lowering the hub assembly. Field balancing of rotating machinery, per se, has long been known. However, windmills rotate at low speeds and this makes the problem of balancing wind turbines somewhat more difficult than balancing high speed machinery. This is because the force exerted due to unbalance is proportional to the square of the angular velocity of the rotor. Therefore, any unbalance in a wind turbine is relatively difficult to detect and correct as compared to high speed machinery. Balancing of low speed rotating machinery is, however, possible. Narrow band spectrum analyzers (FFT analyzers) have been used as instruments for gathering the data necessary for field balancing of rotating masses. Many different manufacturers of narrow band spectrum analyzers now offer digital calculators or computer based programs for field balancing of rotors.
The usual method used to field balance rotors is commonly referred to as the "trial weight method". This method relies on the measurement of the phase of the rotor's unbalance. It can be applied to both single and multiplane balancing problems. In its simplest form a dual channel narrow band analyzer is utilized. Two sensors are used, a vibration monitoring device (accelerometer or velocity transducer) and a tachometer probe. The tachometer probe is used to obtain a phase reading when analyzed in conjunction with the vibration monitoring device in what is commonly referred to as a transfer function. The vibration monitoring device is also used to determine amplitude of vibration induced by the unbalance. The rotating mass is first subjected to measurement in its original state of unbalance at normal operating speed to determine the magnitude of the vibration caused by the unbalance and the location of the unbalance relative to some fixed point on the rotor. This measurement is commonly referred to as the "initial run". A second measurement, commonly referred to as a "trial mass run", is made after the application or removal of a known weight at a fixed radius. The rotor is again rotated at normal operational speed and amplitude and phase data are again collected. The original magnitude and phase data, along with the trial mass run magnitude and phase data and the known trial mass, are used to determine the unbalance magnitude and its phase relation to some fixed point on the rotor at the selected radius. A disadvantage of this method is the necessity to add a trial weight. This weight must be either removed after measuring the unbalance, or balanced out when balancing the rotor if left in place. Therefore time is consumed in adding and removing the weight. In the case that the trial weight is simply balanced out, additional weight must be added to the rotor. The addition of extra balancing mass makes the rotor heavier and also increases the cost of the balancing procedure by the cost of the extra weight added.
Another method which can be used to determine the amount of imbalance in a wind turbine is the so called "four run method." The four run method is an older method of determining unbalance than the "trial weight method." It does not require phase measurement and can, therefore, be performed with less sophisticated instrumentation. In the four run method there is an initial run in which the rotor is rotated at operating speed with no mass added and the magnitude of the imbalance is measured. No phase is measured. Next a known trial weight is added to a point on the rotor at a predetermined radius and the rotor is again rotated with the magnitude of the imbalance being measured with the trial weight in place. Two more measurements of imbalance magnitude are taken with the trial weight in two different angular locations at the predetermined radius. Thus four runs are taken, one with no weight added and three with a trial weight at three different locations on the rotor. The three locations of the trial weights are usually at evenly spaced locations, approximately 120 degrees apart, around the rotor. The results of these four runs can be used in a graphical method or in a matrix analysis to determine the magnitude and phase of the rotor's imbalance and appropriate counterbalance weight can be added. A major disadvantage of the four run method is the amount of time which is consumed in adding and removing the three trial weights at various locations on the rotor. This problem is especially significant when balancing a windmill since the rotor is usually 80 feet or more in the air and the rotor is not adapted for easy addition of the trial weights.